1. Field of the Invention
The present invention relates to a method and to a system for the plasma arc cutting of a workpiece with automatic adaptation of the characteristics of the plasma jet by making corrections, simultaneously and in practically real time, to several parameters, in particular in the complicated portions of the cut path.
2. Related Art
When cutting out of relatively complicated shapes in workpieces, possibly involving sharp changes of direction within the cut paths, for example when making an acute angle in order to form, for example, a pointed profile, it is customary to use a plasma cutting system comprising a plasma cutting torch, a generator for supplying electric current to the torch and to the workpiece, a motorized multiaxis machine for moving the torch relative to the workpiece, or vice versa, along two-dimensional or three-dimensional cut paths, means for programming and controlling the movements of the shafts of the machine, such as a computer numerical control (CNC) console, and means for supplying plasma gas and optionally shielding gas to the plasma torch.
It has actually been found in practice that any sharp change of direction during execution of the cut path requires a change in speed of movement of the torch along the path on the part of the CNC machine so as to ensure that the path executed corresponds exactly to the programmed path.
Thus, with respect to the point or site of a sharp change of direction, there is a first, “deceleration”, region located upstream of said point of sharp change of direction, that has to be taken into account in order to reduce the speed of the movement shaft or shafts in question so as to reach the point of change of direction without overshooting the programmed path profile.
This first region is followed by a second, “acceleration”, region located downstream of the point of sharp change of direction, which also has to be taken into account in order to increase the speed of the movement shaft or shafts in question so that, starting from the resulting speed at the end of deceleration at the point of sharp change of direction, the speed of movement of the torch along the initially programmed path at the end of acceleration is again reestablished.
Moreover, when the machine modifies the initially programmed cutting speed in order to negotiate a change of direction of the cut path, the plasma jet coming from the torch, the characteristics of which were initially adapted and optimized for the initially programmed cutting speed, therefore no longer has its characteristics perfectly adapted to these temporary, new cutting speed conditions, especially from the standpoint of the thermal energy used to locally melt the material and from the standpoint of the kinetic energy used to expel the molten material out of the cut kerf.
This therefore results in the cutting quality deteriorating in the regions where the cutting speed is different from that initially programmed.
By way of example, the deterioration in the cutting quality may be characterized by the formation of burrs that adhere relatively strongly to the base of the cutting kerf and/or by a broadening of the cutting kerf and/or by a loss of perpendicularity of the cut faces and/or by a change in the angle formed by the cut face and the plane formed by the workpiece. This is illustrated in particular in FIG. 2 appended hereto.
To try to solve these problems, document EP-A-1 048 387 proposes a method for adapting the thermal energy of the plasma jet according to the degree of linear advance of the torch and/or to a control parameter proportional to this degree of linear advance, for example the cut diameter of a circular path.
However, this method is not entirely satisfactory, especially as it does not provide for the kinetic energy of the plasma jet to be adapted according to said degree of linear advance.
Consequently, in the absence of such complementary adaptation of the kinetic energy, to solely adapt the intensity of the cutting current according to the degree of linear advance would not preclude the generation of all or some of the aforementioned defects and problems during the cutting operation.
Moreover, to help to keep the cut quality constant over the entire cut path, it is also necessary to be able to keep the torch at an approximately constant distance from the plane formed by the workpiece throughout the time taken to execute the cut path.
For this purpose, plasma cutting machines are generally provided with a motorized Z-axis shaft for moving the torch in a direction perpendicular to the plane (XY axis) formed by the workpiece so as to regulate the distance separating the torch from the plane formed by the workpiece.
This distance is kept approximately constant by a device that continuously measures the voltage of the plasma arc and compares it to a preprogrammed value corresponding to the optimum work conditions. Such methods and devices are described especially in documents WO-A-99/04924 and EP-A-562 111.
When a difference is detected between the measured value and the reference value, depending on the requirement, the motorized Z-axis shaft moves the torch away from or closer to the workpiece so that the measured voltage value is again the reference voltage value.
However, automatic adjustment of this optimum distance according to a measurement of the arc voltage loses its effectiveness when the characteristics of the plasma arc are changed, especially as a result of a change in cutting speed and/or a change in the cutting current and/or a change in the flow rate and/or the pressure of the plasma gas and optionally of the shielding gas.
Beside the case of sharp changes of direction, such as the abovementioned angles, which involve a deceleration followed by an acceleration, there are other cases in which problems of cut quality deterioration occur.
Thus, the problem also arises when plasma-cutting small shapes in metal plates, such as small holes and holes of various, for example round, oblong, etc., shapes, or path portions having fine details, such as curves and filets of small dimensions, for which the CNC machine moves the torch at a speed below the programmed speed, for the same reasons as in the case of cutting angles, namely to comply with the programmed geometry.
In that case, it is not possible to set the minimum radius below which the CNC machine will change the speed, as this depends on the numerical control and on the “machine” parameters, for example the maximum tracking error imposed in the machine program.
However, and as an illustration, if a CNC machine is programmed for a given tracking error and a minimum radius of 50 mm for an actual speed of 10 m/min, any path of a radius or path having a radius of 5 mm will be executed, in that portion, at a maximum speed of 1 m/min (10×5/50), which means that, if the procedure calls for an optimum cutting speed of 3 m/min, there will be defects in the cut in this region.
Starting from this situation, the problem that arises is therefore to improve the known methods and devices, that is to say to be able to prevent the formation of the abovementioned defects and to keep the cut quality substantially constant over the entire perimeter of the cut workpieces, whatever it is, that is to say over the entire cut path, most particularly when the cut path is complicated, for example when it has acute or similar angles, or when shapes of small size or path regions having fine details have to be cut, and to do so irrespective of the speed changes generated by the machine in order to negotiate the cut contours thereof.